How can I find the reason network writes fail with ENOMEM on 7.x?

[Eugene Perevyazko]

I've found a high number of failures in output of `vmstat -z` in category of "NetGraph 
data items".
Trying to monitor those failures I see that they increase in short moments and 
stay at the level for longer.
It's like 
NetGraph data items:       36,      546,        1,      545, 514855314626, 1247384311
NetGraph data items:       36,      546,        6,      540, 514855480968, 1247384311
NetGraph data items:       36,      546,        5,      541, 514855721631, 1247384311
NetGraph data items:       36,      546,      507,       39, 514857074684, 1247384437
NetGraph data items:       36,      546,       22,      524, 514857862740, 1247384443
with a second between consecutive lines.

It looks like I need to raise net.graph.maxdata from a default of 512. 
What are the safe values for this tunable cause it carries comment of "limit the 
damage of a DoS"? What kind of DoS is it about?

-- 
Eugene Perevyazko